Re: [PATCH] Documentation: tpm_tis

["Jarkko Sakkinen" <jarkko@kernel.org>]

On Tue Mar 5, 2024 at 12:53 AM EET, Randy Dunlap wrote:
>
>
> On 3/4/24 13:27, Jarkko Sakkinen wrote:
> > Based recent discussions on LKML, provide preliminary bits of tpm_tis_core
> > dependent drivers. Includes only bare essentials but can be extended later
> > on case by case. This way some people may even want to read it later on.
> > 
> > Cc: Jonathan Corbet <corbet@lwn.net>
> > CC: Daniel P. Smith <dpsmith@apertussolutions.com>
> > Cc: Lino Sanfilippo <l.sanfilippo@kunbus.com>
> > Cc: Jason Gunthorpe <jgg@ziepe.ca>
> > Cc: Peter Huewe <peterhuewe@gmx.de>
> > Cc: James Bottomley <James.Bottomley@HansenPartnership.com>
> > Cc: Alexander Steffen <Alexander.Steffen@infineon.com>
> > Cc: keyrings@vger.kernel.org
> > Cc: linux-doc@vger.kernel.org
> > Cc: linux-kernel@vger.kernel.org
> > Cc: linux-integrity@vger.kernel.org
> > Signed-off-by: Jarkko Sakkinen <jarkko@kernel.org>
> > ---
> >  Documentation/security/tpm/index.rst   |  1 +
> >  Documentation/security/tpm/tpm_tis.rst | 30 ++++++++++++++++++++++++++
> >  2 files changed, 31 insertions(+)
> >  create mode 100644 Documentation/security/tpm/tpm_tis.rst
> > 
>
> > diff --git a/Documentation/security/tpm/tpm_tis.rst b/Documentation/security/tpm/tpm_tis.rst
> > new file mode 100644
> > index 000000000000..3cec0216a169
> > --- /dev/null
> > +++ b/Documentation/security/tpm/tpm_tis.rst
> > @@ -0,0 +1,30 @@
> > +.. SPDX-License-Identifier: GPL-2.0
> > +
> > +=========================
> > +TPM FIFO interface Driver
> > +=========================
> > +
> > +FIFO (First-In-First-Out) is the name of the hardware interface used by the
> > +`tpm_tis_core` dependent drivers. The prefix "tis" is named after TPM
> > +Interface Specification, which is the hardware interface specification for
> > +TPM 1.x chips.
> > +
> > +Communication is based on a 5 KiB buffer shared by the TPM chip through a
> > +hardware bus or memory map. The buffer is further split to five equal size
> > +buffers, which provide equivalent sets of registers for communication
> > +between CPU and TPM. The communication end points are called *localities*
> > +in the TCG terminology.
> > +
> > +When a kernel wants to send a commands to the TPM chip, it first reserves
> > +locality 0 by setting `requestUse` bit in `TPM_ACCESS` register. The bit is
> > +cleared by the chip when the access is granted. Once completed its
> > +communication, it sets `activeLocity` bit in the same register.
>
>       Is that              activeLocality ?

Yes.

>
> > +
> > +Pending localities are served in order by the chip descending orderm and
> > +one at a time:
> > +
> > +- Locality 0 has the lowest priority.
> > +- Locality 5 has the highest priotiy.
>
>                                 priority.
>
> > +
> > +Further information on purpose and meaning of the localities can be found
> > +from section 3.2 of TCG PC Client Platform TPM Profile Specification.

Thanks for the remarks. Too many typos but at least I think the story is
is understandable and describes pretty well key elements of tpm_tis_core.

BR, Jarkko